The present invention relates to a continuously variable transmission system mounted on a vehicle, and more specifically to a belt type continuously variable transmission system for continuously varying a reduction ratio by varying groove widths of pulleys drivingly connected by a belt.
A continuously variable transmission (CVT) system generally includes a CVT mechanism, a forward/reverse changeover mechanism for reversing the rotational direction with friction devices such as clutch and brake, a torque converter between an engine and the CVT mechanism, and a hydraulic fluid pressure control system.
FIG. 16 schematically illustrates a conventional example as disclosed in a Japanese Patent Provisional Publication No. 7(1995)-259941 in a somewhat simplified form. A similar CVT system is disclosed in a U.S. Pat. No. 5,607,373.
The CVT system of FIG. 16 has a series combination of a pump 1 directly connected with a rotation drive system, a line pressure regulating valve 2, a clutch pressure regulating valve 3 and a torque converter pressure regulating valve 4. A lubricating system 9 is connected on the downstream side of the torque converter pressure regulating valve 4. On the upstream side of each regulating valve, a corresponding one of CVT mechanism 6, forward/reverse changeover mechanism 7 and torque converter 8 is connected by a branch line. Line pressure regulating means 2a supplies a control pressure to a pressure increasing side of the line pressure regulating valve 2. Clutch pressure regulating means 3a supplies a control pressure to a pressure increasing side of the clutch pressure regulating valve 3. A lockup control valve 5 is disposed between the upstream side of the torque converter pressure regulating valve 4 and the torque converter 8. Lockup regulating means 5a supplies a control pressure to the lockup control valve 5. The lockup control valve 5 has first and second outlet ports connected, respectively, to release and apply sides of the torque converter 8 to put the torque converter 8 in a non-lockup state for a hydrodynamic drive and in a lockup state for a direct mechanical drive.
Through upstream port and pressure decreasing side pilot port, the line pressure regulating valve 2 receives the operating fluid under pressure from the pump 1, and regulates the pressure until a hydraulic balance is reached between the spool thrust by a return spring and the pressure increasing side control pressure from the line pressure regulating means 2a and the spool thrust by the pressure decreasing side pilot pressure, to produce the line pressure on the upstream side of the upstream port. This line pressure is supplied to the CVT mechanism 6, specifically to a pulley cylinder chamber. The control pressure from the line pressure regulating means 2a is adjusted so as to prevent slippage of the belt while attaining a target reduction ratio. Therefore, the line pressure supplied to the CVT mechanism 6 is regulated in accordance with an input load on the CVT mechanism 6 such as the engine output (torque) and the reduction ratio of the pulleys.
The remainder of the fluid under pressure reduced by the line pressure regulating valve 2 is supplied to the upstream port and pressure decreasing side pilot port of the clutch pressure regulating valve 3. The clutch pressure regulating valve 3 produces the clutch pressure by regulating the fluid pressure until a balance is reached between the thrust by the return spring and the pressure increasing side control pressure from the clutch pressure regulating means 3a and the thrust by the pressure decreasing side pilot pressure; and supplies the clutch pressure to the forward/reverse changeover mechanism 7. In the system of the above-mentioned Japanese publication, the control pressure of the clutch pressure regulating means 3a is controlled minutely to prevent the creep. However, such a minute control is not required when the system employs the torque converter 8 as mentioned later. To facilitate understanding, therefore, it is possible to consider that the control pressure is adjusted only to meet the demand of each friction device in the forward/reverse changeover mechanism 7.
The remainder of the fluid after the clutch pressure regulating valve 3 is supplied to the upstream port and pressure decreasing side pilot port of the torque converter pressure regulating valve 4. The converter pressure regulating valve 4 produces the torque converter pressure by reducing the fluid pressure until a balance is reached between the thrust by the return spring and the thrust by the pressure decreasing side pilot pressure; and supplies the thus-produced torque converter pressure to the torque converter 8. The remaining fluid is supplied to the lubricating system 9.
The torque converter pressure is supplied from the converter pressure regulating valve 4 through the lockup control valve 5 to the torque converter 8. When the control pressure is not supplied from the lockup regulating means 5a, the lockup control valve 5 supplies the torque converter pressure to the release side and puts the torque converter in the non-lockup state. When the control pressure is supplied from the lockup regulating means 5a, the lockup control valve 5 is held in the position to supply the torque converter pressure to the apply side, and puts the torque converter 8 in the lockup state. The lockup regulating means 5a controls its control pressure mainly in accordance with the vehicle speed and the engine speed. In changing the torque converter from the non-lockup state to the lockup state, the conventional system disclosed in the above-mentioned Japanese publication is arranged to vary the control pressure of the lockup regulating means 5a gradually in accordance with a deviation between the input speed (equivalent to the engine speed) and the output speed (equivalent to the vehicle speed), that is, a deviation between an impeller speed and a turbine speed of the torque converter. The illustrated example shown in FIG. 16 is simplified to facilitate understanding so that the control pressure of the lockup regulating means 5a is an on/off signal to simply alternating between the lockup state and the non-lockup state.
In this pressure control circuit, the fluid from the pump 1 is supplied sequentially to the components in order of required pressure level, so that one component passes the fluid to the next component of lower required pressure level. Such a circuit is efficient and advantageous in flow balance, as compared to a pressure control circuit for an ordinary automatic transmission in which the line pressure is reduced sequentially with restrictions. Specifically in the belt type CVT control system configured to control the fluid pressure supplied to the pulleys in a wide range (with a gain of a considerable magnitude) so as to grip the belt with the pulleys and to vary the groove widths of the pulleys in accordance with the input load to the CVT mechanism, the pressure control circuit of the type shown in FIG. 16 is advantageous because of its capability of readily producing the required pressures for the pulleys and other components. The use of the torque converter may eliminate the necessity of the clutch pressure regulating valve in some cases.
In the conventional system, the torque converter pressure regulating valve is arranged to regulate the torque converter pressure at a constant pressure level. The torque converter pressure is set at the constant pressure level which is below a withstanding pressure defined by a mechanical limit of the torque converter, but which is high enough to secure the lockup state of the torque converter without slippage even in a high output high speed vehicle operation in which the engine is producing a great output and the vehicle speed is high. The conventional system holds the torque converter pressure at the constant level even in a start accelerating operation in which the torque converter is in a non-lockup state, and the difference between the input and output speeds (the impeller speed and turbine speed) of the torque converter increases widely. In the non-lockup state, the torque converter pressure supplied to the release side is drained from the apply side through the clearance between the lockup facing member and the torque converter cover, so that the pressure difference between the release and apply sides is small.
In the above-mentioned high output starting operation (stall start operation), the engine speed is high with deep depression of the accelerator pedal, but the turbine speed equivalent to the wheel speed is still very low. In this state, the narrow pressure difference between the release and apply sides cannot produce a sufficient flow of the fluid between the lockup facing member and the converter cover. Therefore, the clearance therebetween is reduced, and the lockup facing member connected with the turbine (and the wheels) tends to drag the torque converter cover rotating at a high speed with the engine, and to reduce the stability of the engine rotation. In the system controlling the line pressure in accordance with the input load to the CVT mechanism, increasing the line pressure in the non-lockup state and decreasing the line pressure in the lockup state, it is possible to avoid the drag of the lockup member by normally decreasing the line pressure with a restriction and increasing the line pressure in the non-lockup state. In the belt CVT system requiring a large gain control of the line pressure for the pulleys, the arrangement of the restriction cannot fulfil the requirements for the torque converter pressure.
The pressure regulating valves have a construction as shown in FIG. 17A. In the valve of FIG. 17A, a return spring pushes a spool leftwards, and the fluid from an upstream port flows through a groove formed between left and right lands of the spool, to a downstream port. A pilot pressure acts on a pressure receiving are of the left end of the spool. In this type of a pressure reducing valve, an increase in the supply fluid flow causes an increase in displacement of the spool and hence an increase in reaction force of the return spring. As the supply flow Q increases, therefore, the regulated pressure P1 on the upstream side increases beyond a preset pressure level as shown in FIG. 17B. This phenomenon is known as override. The torque converter pressure is set at the relatively high level to ensure the lockup state, and accordingly, the torque converter pressure does not differ so much from the clutch pressure on the upstream side of the clutch pressure regulating valve 3. In the clutch pressure regulating valve, therefore, the little difference between the torque converter pressure on the downstream side and the clutch pressure on the upstream side increases the difficulty in fluid flow between the upstream and downstream sides, and increases the undesired override by increasing the displacement of the spool. Furthermore, the increase in the clutch pressure reduces the pressure difference between the upstream and downstream sides of the line pressure regulating valve 2. This small pressure difference increases the override of the valve 2, and increases the upstream pressure of the valve 2, that is, the downstream pressure of the pump 1. The increase in the pump discharge pressure means an increase of the load on the pump and an increase of the load on the engine for driving the pump, eventually resulting in deterioration in fuel consumption. A decrease of the preset level for the torque converter pressure may avoid this problem by increasing the difference from the clutch pressure. However, the low torque converter pressure cannot ensure the lockup state in the high output high speed operation.